The invention relates to a coil provided with current-conducting turns and with cooling means. The invention also relates to an actuator in which such a coil is used.
Such cooled coils are generally known. The heat removal from the turns may take place, for example, by means of a cooling channel through which a cooling liquid is pumped, see e.g. EP 0414927 and DE 331707, or by means of radiation plates as known from U.S. Pat. No. 5,164,262. The cooling effect of these coils, however, is limited and certainly not ideal. The problem of the cooling is even greater if coils are used in high-power equipment. The removal of heat then becomes highly essential.
It is an object of the invention to provide a coil with a very efficient cooling.
The coil according to the invention is for this purpose characterized in that the coil is provided with a number of heat-conducting foil turns whose edge portions are cooled by the cooling means. Providing the coil with heat-conducting foil turns and cooling the edge portions thereof leads to a good heat transport to the cooled edge portions.
The edge portions could be cooled, for example, by means of an air flow. It is more efficient, however, to connect the edge portions thermally to the cooling means, i.e. to provide a physical connection through which a sufficient heat flow can be conducted. The cooling means used may be a cooling channel through which a cooling liquid is pumped. The edge portions of the foil turns must make good contact with a wall of the cooling channel.
In a coil which is to be used by preference, the heat-conducting foil turns are at the same time the current-conducting turns. The advantage of foil coils over wire coils is the greater filling factor of the current conductors, usually of copper, whereby a much higher efficiency is achieved. Many foil turns are present in each foil coil, all contributing to a very good heat dissipation. It is nevertheless also possible to construct the current-conducting turns as electrically insulated wire turns, between which the heat-conducting turns extend. Depending on the application or the available control, the I-V characteristic can be better optimized with a wire-wound coil than with a foil-wound coil. In addition, the risk of eddy currents arising is smaller in wire-wound coils than in foil-wound coils. The air gaps and the insulation provide a considerable heat barrier in normal wire-wound coils. The provision of heat-conducting turns between the coil wires and cooling of the edge portions of said turns strongly improves the removal of heat. In this latter embodiment, it is useful and sometimes imperative that the heat-conducting turns should not form a closed circuit. This is to prevent the generation of undesirable eddy currents. This objective may be achieved in that the heat-conducting foil turns are interrupted.
Preferably, the cooled coils in accordance with the invention described above may be used in an actuator which is characterized by two systems of permanent N- and Z-magnets situated one above the other, between which the coil according to the invention is accommodated, the direction of a magnetic field generated by the magnets being perpendicular to the plane in which the foil turns are oriented, while the coil is movable with respect to the magnet systems in a direction parallel to the plane in which the foil turns are oriented. High powers can be used in such an actuator, also denoted X-Y actuator. The coil may be very flat, and the air gaps between the coil and the magnets can be small, so that a highly homogeneous magnetic field can be used.
In an alternative embodiment, the coils according to the invention may be used in an actuator which is characterized by one system of alternate permanent N- and Z-magnets above which a coil according to the invention is present, the direction of a magnetic field generated by the magnets being parallel to the plane in which the foil turns are oriented, while the coil is movable with respect to the system of magnets in a direction perpendicular to the plane in which the foil turns are oriented.
The actuators mentioned above may be used as a displacement device in a component placement machine for placing components on a printed circuit board, or in a lithographic device (wafer stepper) for manufacturing integrated circuits on a semiconductor substrate.